I wonder why it's not Elon himself? He can obviously afford it, and it is clearly his passion, so why not take a once in a lifetime opportunity and head into space?
Can he? He definitely has the potential to be massively wealthy. If Tesla/SpaceX/BoringCo/etc exit at some point he'll walk away with '*&^@ you money', but until then he doesn't seem to have that much. According to Bloomberg he's in debt with personal loans to the tune of hundreds of millions of dollars[1].
To be honest though, I don't really understand how normal money-you-buy-stuff-with works for people who are billionaires when most of their wealth is in stock and options.
[1] https://www.bloomberg.com/news/articles/2018-08-16/musk-pick...
He takes out loans with his shares as collateral to get spending money.
Why not just sell? Because he expects the shares to appreciate more than what he'd pay in interest and/or to retain the shareholder vote.
The super rich don't have a lot in liquid cash, it's all tied up.
And more importantly, the risk:reward of failure involving Elon is bad. Right now he's managed to spark a number of competitors such as Blue Origin and Virgin Galactic, but these companies have yet to achieve much of anything and it's not out of the question to consider that if Elon died tomorrow, we might enter into another 50 years of space stagnation. Contrast that against the pleasure gained from a space voyage and it's just a bad risk:reward ratio, even when the risk is mostly negligible.